The present invention relates to a ball bearing used in various motors, which are used for a general industrial purpose. More particularly, the invention relates to a ball bearing which reduces the noise, vibration, fretting damage (abrasion) and torque of the ball bearing, and improves the acoustic durability of the cage noise.
An example of such for the general industrial purpose is a motor device for driving an air conditioning apparatus (referred to as an air conditioner). Recently, the air conditioner has been improved to have high performances and a multifunctionality. The air conditioner is operated in the following way, for example. Under control of the inverter unit, the air conditioner is operated at high speed to effect a rapid cooling and to lower room temperature in the shortest possible time. Then, it is operated at low speed to keep the room at constant temperature. In such circumstances, when the air conditioner is operated at the low speed, low noise operation is required, viz., it is operated generating reduced noises of the air blowing, the motor rotation and the like. In the low speed operation, the cooling efficiency within the apparatus decreases, the temperature of the rolling bearing incorporated into the motor increases to sometimes reach 100 to 120xc2x0 C. In this condition, it is difficult to secure a thickness of the oil film formed by the lubrication, the grease packed in the ball bearing is liable to deteriorate. With progress of the deterioration, noise will be generated.
In the outdoor unit used for the air conditioner, the initial noise (cage noise) of the bearing sometimes is required to reduce at the start operation in low temperature environment, e.g., in winter season. Furthermore, the motor units of air-conditioners with rolling bearings installed therein are sometimes transported over long distances from a motor manufacturer to end users by a truck. In such a long distance transportation, the truck traces the fine unevenness of roads, and the associated movement is transmitted to the rolling bearing in the form of repetitive impact loads, which cause the rolling element of the rolling bearing to repeatedly put into microscopic contact with the raceway surface. Such microscopic contacts sometimes cause a fretting damage (abrasion) on the raceway surface, and the damage acts as a source of noise.
Taking account of environmental regulations as well as the achieving of high performances and multifunctionality, efforts have been made to downsize the motor and to realize the lower output of the motor in order to reduce the heat generation of the motor. For this reason, it is recognized that the torque characteristic has a vital function in the rolling bearings having those uses. The dynamic frictional torque of the rolling bearing is caused by the frictions by a minute slip on the rolling contact surface, sliding friction at the sliding contact part in the bearing, and viscosity resistance of the grease. It is known that the viscosity resistance of the grease is affected by the kinematic viscosity of the base oil and the worked penetration of the grease. Accordingly, the kinematic viscosity of the base oil is based on a shearing resistance of the oil when a fluid lubricating film is formed. In this sense, reduction of the kinematic viscosity plays an important role in reducing the dynamic frictional torque of the rolling bearing. The grease worked penetration affects the channeling performance when it is sheared within the bearing when the bearing rotates. In this sense, the approach of reducing the worked penetration of the grease presents an effective solution to the reducing of the dynamic frictional torque of the rolling bearing.
When the kinematic viscosity of the base oil is reduced, because sometimes the motor of the air conditioner is rotated at relatively low speed under the inverter control, it is difficult to secure the thickness of the oil film. Generally, the oil of low kinematic viscosity is low in heat resistance, and a problem arises in the acoustic durability. The reduction of the worked penetration of the grease results in the addition amount of the thickener. In this case, the amount of the base oil decreases relative to the amount of other compositions in the grease, and the resisting force of the grease to the mechanical shear increases. As a result, an amount of supplying of the base oil to the lubricating surface of the bearing reduces, and it is impossible to stably maintain the lubricating property of the oil for a long term.
Thus, there is a limit in reducing the kinematic viscosity and the worked penetration of the grease. In the rolling bearing having the uses mentioned above, the following values are considered appropriate: the kinematic viscosity of the base oil is 10 to 500 mm2/s at 40xc2x0 C., the worked penetration of the grease is NLGI grades No. 2 to 3, or the thickener is contained in an amount of 5 to 20% by mass based on the total amount of the grease composition. Particularly, in the motor which requires the low noise characteristic, i.e., acoustic durability, a grease is generally used which prepared by blending a fatty acid lithium salt as a thickener into an ester as a base oil. The heat resistance of an ester oil is higher than that of the mineral oil. The ester oil contains a polar group in the molecular structure. The polar group functions to increase the sorbability to the metallic surface, to improve the friction characteristic and the acoustic durability. Further, where the reduction of the fretting damage (abrasion) is required, it is effective to use a base oil of high oil film forming property and relatively high viscosity.
For example, a ball bearing as shown in FIG. 1 is known for the rolling bearing having the uses stated above. The ball bearing is constructed such that an inner ring 2 having inner raceway 1 formed on its outer peripheral surface and an outer ring 4 having an outer raceway 3 formed on its inner peripheral surface are coaxially disposed. A plurality of balls 5, 5 are rotatably disposed between the inner raceway 1 and the outer raceway 3. In the illustrated case, the inner raceway 1 and the outer raceway 3 are both of the deep groove type. The balls 5, 5 are rotatably retained in pockets 7, 7 provided in a cage 6.
The cage 6 is called a wave press cage (a corrugated press cage). To form the cage 6, a metal plate is molded into an element 8, wavy and annular in shape, by press molding, and a couple of elements 8, 8 so shaped are combined. Concave parts 9, 9, semicylindrical in shape, are formed at a plurality of positions on the element 8 as viewed circumferentially. Those concave parts 9, 9 are used for forming the pockets 7, 7. The couple of elements 8, 8 are butted against each other at positions apart from the concave parts 9, 9 thereof. Those butted parts are joined and fixed by a plurality of rivets 10 to thereby form a cage 6, annular in shape. The thus formed cage 6 includes pockets 7, 7 arranged at plural positions as viewed circumferentially. A middle part of the inner surface of the concave part 9 is spherically concaved to be arcuate in cross section, with the radius of curvature being slightly larger than that of the outer surface of each ball 5. Accordingly, when the couple of elements 8, 8 are butted against each other, the concave parts 9, 9 are combined to form a pocket 7.
When the ball bearing is used, the inner ring 2 and the outer ring 4 are rotatable relative to each other with the rolling of the balls 5, 5. The balls 5, 5 revolve around the inner ring 2 while rolling about its axis. The cage 6 moves (rotates) around the inner ring 2 at a speed equal to the revolving speed of each ball 5.
A space between the outer peripheral surface of the inner ring 2 and the inner peripheral surface of the outer ring 4 is packed with a lubricant, such as grease or another lubricant, to thereby make the relative rotation smooth. The lubricant of the space prevents the ball bearing from being vibrated or generating noise, and further prevents such a trouble as the seizure from occurring. In the ball bearing for the air conditioner, both end openings of a space between the outer periphery surface of the inner ring 2 and the inner periphery surface of the outer ring 4 are closed by sealing members, such as sealing plates or shielding plates, thereby preventing a lubricant from leaking from the space or preventing foreign matter, e.g., dust, from entering the space. The ball bearing not having such sealing members is illustrated in FIG. 1.
The radii of curvature of the cross-sectional shapes of the inner and outer raceways can be designed in various manners depending on the level of load and rotating speed. But usually the radius of curvature of the inner raceway as well as the outer raceway is made equal to 52% of the ball diameter. This is based on the fact that, in Interpretative Table 2 xe2x80x98Radii of raceway grooves and decreasing coefficientxe2x80x99 of xe2x80x9cMethod of Calculating Dynamic Load Rating and Standard Life of Rolling Bearingxe2x80x94Interpretationxe2x80x9d in JIS standard (JIS B 1518-1992), the radius of curvature of the cross-sectional shape is made equal to 52% of the diameter of the rolling element for the calculation of the dynamic load rating of a deep groove ball bearing. Also, in the bearing catalogue published by the present applicant, dynamic and static load rating are calculated with the assumption of the radii of curvature of the cross-sectional shapes of the inner raceway and the outer raceway being 52% of the diameter of the rolling element. As is seen from these facts, the radii of curvature of the cross-sectional shapes of the inner raceway and the outer raceway are usually made equal to 52% of the diameter of the rolling element.
In the case of the ball bearing mentioned above, even if it is packed or supplied with a necessary amount of lubricant, the cage 6 is caused to vibrate. This leads to generation of noise called cage noise and a vibration of the cage. Such a vibration of the cage 6 is caused by a sliding friction between the balls 5, 5 and the cage 6, which is due to the fact that an amount of motion of the cage 6 is larger than that of the balls 5, 5. A measure conventionally taken for suppressing the cage noise is to reduce the motion amount of the cage 6 to the balls 5, 5 by reducing a clearance between the inner surface of the pockets 7, 7 and the rolling surface of the balls 5, 5.
However, sometimes the cage noise generation is still present even if the amount of motion of the cage 6 to the balls 5, 5 is reduced when the operating condition is severe, for example, when an insufficient amount of lubricant is supplied. This is due to a configuration of the inner periphery surface of the pocket 7 of the cage 6. Specifically, in the conventional cage 6 shown in FIG. 1, the inner peripheral surface of the pocket 7 may be in sliding contact with the rolling surface of the ball 5 over the substantially entire surface of the pocket. Therefore, a friction force acting on the inner periphery surfaces of those pockets and the rolling surfaces of the balls is large. This will be described in detail with reference to FIGS. 20 and 21.
In the conventional structure of the cage shown in FIG. 1, as indicated by tilt lattice in FIGS. 20 and 21, most of each concave part 9 of the inner peripheral surface of each pocket 7 serves, over its substantially full width, as a spherical part 15 which functions as a holding guide surface the radius of curvature of which is somewhat larger than that of the rolling surface of the ball 5 (FIGS. 1 and 23). When the inner peripheral surface of the pocket 7 is formed, over its full width, as the spherical part 15 functioning as the holding guide surface, the friction area between the inner peripheral surface of the pocket 7 and the rolling surface of the ball 5 is increased. A frictional vibration generated at a sliding contact part between the cage 6 and the ball 5 grows, thereby giving rise to vibration and noise. In a case where each pocket 7 is configured over its full width as a spherical part 15 as a single spherical surface, if the center O15 (FIG. 23) of each spherical part 15 of each pocket is set off from the center O5 (FIGS. 23 and 24) of the ball 5 as retained in the pocket 7, the lubricant sticking to the rolling surface of the ball 5 is scraped from the rolling surface, and the vibration and the noise remarkably grows. This will be described in detail with reference to FIGS. 22 to 24 by using a wave press cage as shown in FIG. 1.
In the conventional cage 6, the spherical parts 15 defining each pocket 7 are each configured as a single spherical surface where the radius of curvature R15 of the spherical part 15 is somewhat larger than that R5 of the ball 5 as shown in FIG. 22 (R15 greater than R5). A depth D7 of the pocket 7, xc2xd as large as the inner dimension of the pocket 7 as viewed in the widthwise direction of the cage 6, is slightly smaller than the radius of curvature R15 of the spherical part 15 as exaggeratedly illustrated in FIG. 23.
In the ball bearing with such a cage 6 incorporated thereinto, when it is operating, the rolling surface of each ball 5 comes in contact with the inner peripheral surface of each pocket 7 in the cage 6. Those balls revolve while rolling at the same speed as of the cage 6. The revolving speeds of those balls 5 are not exactly equal to one another, viz., those balls revolve while being minutely lagged and advanced. This is due to shape errors of the inner raceway 1 and the outer raceway 3 (FIG. 1), diametrical size variation of the balls per se, an inclination of the ball bearing (offset between the center axes of the inner ring 2 and the outer ring 4), and others. As a result, some balls 5 push the cage 6 in the revolving direction, while the cage 6 pushes the balls 5. In either case, the rolling surfaces of the balls 5 come in contact with the spherical parts 15 defining the inner peripheral surfaces of the pockets 7. That is, since the radius of curvature R15 of the spherical part 15 is larger than that the radius of curvature R5 of each ball 5, the cage 6 displaces in the radial direction a distance corresponding to a clearance caused by the difference between the radii of curvature R15 and R5, as shown in FIG. 23. In this condition, the rolling surface of each ball 5 comes in sliding contact with the spherical parts 15 defining each pocket 7. Specifically, as shown in FIGS. 23 and 24, the spherical parts 15 defining each pocket 7 are brought into sliding contact with the rolling surface of the ball 5 on both sides of the cage 6 as viewed in the widthwise direction of the cage 6 (vertical direction in FIG. 23 and horizontal direction in FIG. 24) at two points P1 and P2 each displaced from the center of the pocket 7 as viewed in the circumferential direction to the ends as viewed in the circumferential direction.
When the center O7 of the pocket 7 of the cage 6, as shown in FIG. 24, is set off from the center O5 of the ball 5 toward the inner side owing to the clearance caused by the difference between the radii of curvature R15 and R5, a part of the rolling surface of the ball 5, which is closer to the outer diameter of the cage 6 comes in sliding contact with a part of the spherical part 15 forming the inner peripheral surface of the pocket 7, which is close to the outer diameter of the cage 6. Accordingly, the lubricant, such as grease or oil, which is supplied for lubricating the ball bearing and sticks to the rolling surface of each ball 5, is scraped off with the edge of the spherical part 15, and the scraped one is pushed outside while not taken into the pocket 7. On the other side of the cage 6 as viewed in the circumferential direction, a part of the rolling surface of the ball 5, closer to the inner diameter of the cage 6 is in sliding contact with a part of the spherical part 15 defining the inner peripheral surface of the pockets 7, closer to the inner diameter of the cage 6. Also in this case, the pocket fails to take the lubricant thereinto.
As the result of the lubricant taking-in failure, the coefficient of sliding friction at the sliding part between the rolling surface of each ball 5 and the spherical part 15 defining the inner peripheral surface of each pocket 7 of the cage 6, increases. When the friction coefficient has increased, a frictional torque of the ball bearing having the cage 6 assembled thereinto will vary or increase. Further, a friction noise is generated at the time of operation. In some cases, the friction noises will greatly grow.
Accordingly, an object of the present invention is to provide a ball bearing which succeeds in reducing the noise, vibration, fritting damage (abrasion) and torque of the ball bearing, and in improving the acoustic durability of the cage noise.
To achieve the object, the invention provides the following ball bearings of the first to fourth aspects.
(1) A ball bearing comprising an outer ring having on its inner periphery an outer raceway, an inner ring having on its outer periphery an inner raceway, plural balls rotatably disposed between the outer raceway and the inner raceway, a cage holding the plural balls in a freely rotatable manner, and a grease packed in a space between the outer raceway and the inner raceway, wherein the grease comprises a base oil comprising a lubricant having a polar group in its molecular structure and a non-polar lubricant and a metallic soap-based thickener containing a long fibrous material whose fiber length is at least 3 xcexcm in an amount of at least 30% by mass based on the total amount of the thickener; the inner peripheral surface of each pocket of the cage comprises a spherical part consisting of a spherical concave surface whose radius of curvature is slightly larger than that of each ball and a curved part which has the radius of curvature being larger than that of the spherical part and is smoothly continuous from the end of the spherical part toward the end thereof on the opening side of the pocket; and the radius of curvature of the cross-sectional shape of the inner raceway and the radius of curvature of the cross-sectional shape of the outer raceway are in the range of from 51.0% to smaller than 60.0% of the diameter of each ball.
(2) A ball bearing comprising an outer ring having on its inner periphery an outer raceway, an inner ring having on its outer periphery an inner raceway, plural balls rotatably disposed between the outer raceway and the inner raceway, a cage holding the plural balls in a freely rotatable manner, and a grease packed in a space between the outer raceway and the inner raceway, wherein the grease comprises a base oil and a metallic soap-based thickener containing a fibrous material whose fiber length is at least 3 xcexcm in an amount smaller than 30% by mass based on the total amount of the thickener; the inner peripheral surface of each pocket of the cage comprises a spherical part consisting of a spherical concave surface whose radius of curvature is slightly larger than that of each ball and a curved part which has the radius of curvature being larger than that of the spherical part and is smoothly continuous from the end of the spherical part toward the end thereof on the opening side of the pocket; and the radius of curvature of the cross-sectional shape of the inner raceway is in the range from 51.5% to 56.0% of the diameter of each ball and the radius of curvature of the cross-sectional shape of the outer raceway is in the range from 52.5% to 58.0% of the diameter of each ball.
(3) A ball bearing comprising an outer ring having on its inner periphery an outer raceway, an inner ring having on its outer periphery an inner raceway, plural balls rotatably disposed between the outer raceway and the inner raceway, a cage holding the plural balls in a freely rotatable manner, and a grease packed in a space between the outer raceway and the inner raceway, wherein the grease comprises a base oil comprising a lubricant having a polar group in its molecular structure and a non-polar lubricant, and a metallic soap-based thickener containing a long fibrous material whose fiber length is at least 3 xcexcm in an amount of at least 30% by mass based on the total amount of the thickener; the cage is in the shape of circular ring as a whole and comprises pockets for receiving and holding the respective balls with its pocket surface at plural places in its circumferential direction and an opening whose width is smaller than the diameter of the ball on one side of the axial direction of each pocket, while a ratio xcex4/Da of an axial clearance xcex4 provided between the rolling surface of the ball in the axial direction and the pocket surface to the diameter Da of each ball is set in a range from xe2x88x920.01 to 0.02; and the radius of curvature of the cross-sectional shape of the inner raceway and the radius of curvature of the cross-sectional shape of the outer raceway are in the range from 51.0% to smaller than 60.0% of the diameter of each ball.
(4) A ball bearing comprising an outer ring having on its inner periphery an outer raceway, an inner ring having on its outer periphery an inner raceway, plural balls rotatably disposed between the outer raceway and the inner raceway, a cage holding the plural balls in a freely rotatable manner, and a grease packed in a space between the outer raceway and the inner raceway, wherein the grease comprises a base oil and a metallic soap-based thickener containing a fibrous material whose fiber length is at least 3 xcexcm in an amount smaller than 30% by mass based on the total amount of the thickener; the cage is in the shape of circular ring as a whole and comprises pockets for receiving and holding the respective balls with its pocket surface at plural places in its circumferential direction and an opening whose width is smaller than the diameter of the ball on one side of the axial direction of each pocket, while a ratio xcex4/Da of an axial clearance xcex4 provided between the rolling surface of the ball in the axial direction and the pocket surface to the diameter Da of each ball is set in a range from xe2x88x920.01 to 0.02; and the radius of curvature of the cross-sectional shape of the inner raceway is in the range from 51.5% to 56.0% of the diameter of each ball and the radius of curvature of the cross-sectional shape of the outer raceway is in the range from 52.5% to 58.0% of the diameter of each ball.
The ball bearing of the first or second aspect of the present invention contains a cage constituted as above. In the cage thus constructed, the inner peripheral surface of each pocket rubs on the rolling surface of each ball only on the spherical part, not on the curved parts. A clearance is present between the curved part and the rolling surface, and is larger than the clearance between the spherical part and the rolling part of the ball. Accordingly, a friction area between the inner peripheral surface of each pocket and the rolling surface of the ball is reduced, and further the lubricant (grease) is smoothly and effectively taken into the clearance between the spherical part and the rolling surface. With those operations, a friction acting on the sliding contact part between the cage and the ball is more lessened. A frictional vibration generated at the sliding contact part is lessened, and the vibration and noise are lessened.
By making the radius of curvature of the cross-sectional shape of the inner raceway 51.5% to 56.0% of the diameter of the ball, and the radius of curvature of the cross-sectional shape of the outer raceway 52.5% to 58.0% of the diameter of the ball in the ball bearing of the above-described first aspect of the present invention and the ball bearing of the above-described second aspect of the present invention, the elastic deformation of the contact portion in the rolling surface of the ball with the inner raceway surface or the outer raceway surface is reduced. In other words, the Hertian contact ellipse becomes smaller, thus the differential slippage is reduced, resulting in the reduction of bearing torque. Simultaneously, against the impact loading repetitively applied during transportation, fretting damages (abrasion) can be reduced which occur at the inner raceway surface or the outer raceway surface, leading to the improvement of acoustic durability. With a radius of curvature of the cross-sectional shape of the inner raceway or the outer raceway larger than the upper limit mentioned above, the maximum Hertian contact pressure at the contact elliptic area becomes too large, thus shortening the rolling fatigue life of the inner raceway or the outer raceway. Hence, an unfavorable acoustic property and a short flaking life result. By making the radius of curvature of the cross-sectional shape of the outer raceway larger than the radius of curvature of the cross-sectional shape of the inner raceway, the difference of the contact surface pressure between the rolling surface of the ball and the inner raceway or the outer raceway can be decreased.
In the ball bearing of the above-described first aspect of the invention, bearing torque can be reduced due to the use of the above-specified grease composition. The long fibrous material having a fiber length of at least 3 xcexcm contained in the thickener for the grease composition in an amount of 30% by mass or larger is oriented by the shearing force exerted by the rotation of the bearing, and acts to reduce the bearing torque. In particular, in the ball bearing of the above-described first aspect, this bearing torque-reducing effect is further enhanced by jointly using a non-polar lubricant in the base oil. Further, the lubricant having a polar group in its molecular structure (which will be called polar group-containing lubricant hereinafter) contained in the base oil plays a role similar to that of the conventional polar group-containing base oil (such as ester oils). The polar group is preferentially adsorbed on the contact surface for the rotary part of the bearing to form an adsorption film, which reduces the bearing torque through the improvement of surface frictional property. Still further, the polar group-containing lubricant interacts with the micelle structure of the metallic soap to reduce the bonding force between the long fibrous materials. Hence, the shear resistance of the grease during bearing rotation is reduced, resulting in a further reduction of bearing torque. Because of such mechanisms, even if the radius of curvature of the cross-sectional shape of the inner raceway or the outer raceway is made smaller than 52% of the diameter of the ball, an improved dynamic torque property is achieved compared to the property achieved in conventional specifications provided that the radius of curvature is at least 51% of the diameter of the ball.
Since the ball bearing of the above-described first or second aspect of the invention can reduce the friction acting between the raceway surface of the ball and the outer raceway and the inner raceway during the relative revolution of the outer ring and inner ring, heat generation inside the ball bearing can be effectively suppressed. As a result, the deterioration of the grease composition packed in the bearing is prevented to secure improved acoustic durability for the ball bearing.
On the other hand, the ball bearings of the above-described third or fourth aspect of the invention are the same in constitution as the ball bearings of the above-described first or second aspect of the invention, respectively, except for incorporating a cage comprising pockets each having a specific shape, so-called xe2x80x9ccrown-type cagexe2x80x9d. Accordingly, the ball bearings of the above-described third or fourth aspect of the invention take the same effects as those of the ball bearings of the above-described first or second aspect of the invention, respectively